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Sleep stages are characterized by spectral content of EEG: for instance, stage N1 refers to the transition of the brain from alpha waves (common in the awake state) to theta waves, whereas stage N3 (deep or slow-wave sleep) is characterized by the presence of delta waves. [107] The normal order of sleep stages is N1 → N2 → N3 → N2 → REM.
Delta wave activity, correlating with slow-wave (deep) sleep, in particular shows regular oscillations throughout a good night's sleep. Secretions of various hormones, including renin, growth hormone, and prolactin, correlate positively with delta-wave activity, while secretion of thyroid-stimulating hormone correlates inversely. [3]
Slow-wave sleep (SWS), often referred to as deep sleep, is the third stage of non-rapid eye movement sleep (NREM), where electroencephalography activity is characterised by slow delta waves. [ 2 ] Slow-wave sleep usually lasts between 70 and 90 minutes, taking place during the first hours of the night. [ 3 ]
K-complexes are single long delta waves that last for only a second. [10] They are also unique to NREM sleep. They appear spontaneously across the early stages, usually in the second stage, much like the sleep spindles. However, unlike sleep spindles, they can be voluntarily induced by transient noises such as a knock at the door.
Gamma waves are seen when a person is highly focused on a task or using all their concentration. Theta waves occur during the period of a person being awake, and they continue to transition into Stage 1 of sleep and in stage 2. Delta waves are seen in stages 3 and 4 of sleep when a person is in their deepest of sleep. [17]
During slow wave sleep, the cortex generates brief periods of activity and inactivity at 0.5–4 Hz, resulting in the generation of the delta waves of slow wave sleep. During this period, the thalamus stops relaying sensory information to the brain, however it continues to produce signals, such as spindle waves, that are sent to its cortical ...
The activity of K-complexes is transferred to the thalamus where it synchronizes the thalamocortical network during sleep, producing sleep oscillations such as spindles and delta waves. [7] It has been observed that they are indeed identical in the "laminar distributions of transmembrane currents" to the slow waves of slow-wave sleep. [1]
Brainwave entrainment, also referred to as brainwave synchronization or neural entrainment, refers to the observation that brainwaves (large-scale electrical oscillations in the brain) will naturally synchronize to the rhythm of periodic external stimuli, such as flickering lights, [1] speech, [2] music, [3] or tactile stimuli.